The present invention relates to a system for providing an indication of the acceleration experienced by a vehicle in the plane of the surface over which the vehicle moves and, more particularly, for providing such an indication for a vehicle of the type having a suspension system which permits the vehicle body to pitch and roll.
The dynamic forces which act upon a vehicle, such as an automobile, as it travels over an arbitrary course are of particular interest to race drivers, vehicle designers, tire manufacturers, and others. Particularly meaningful is acceleration data with respect to the lateral axis and forward axis of the vehicle, taken in the plane of the road, track, or other surface over which the vehicle travels. Such acceleration data can be presented visually in a "g--g diagram," as suggested by R. S. Rice in SAE paper No. 730018, January 1973, A g--g diagram is a plot of acceleration showing lateral acceleration along a horizontal axis and forward acceleration along a vertical axis, with successive points on the plot defining a path which illustrates the acceleration experienced over time.
Regardless of the manner in which acceleration data is displayed, one significant difficulty in obtaining meaningful data for a land vehicle, such as an automobile, is that the measurement accelerometers are generally mounted within the passenger compartment of the vehicle body. The vehicle body, in turn, is supported on a suspension system which permits limited roll and pitch to enhance passenger comfort and vehicle handling. As the vehicle is driven over an arbitrary course, the measurement axes of the accelerometers in the vehicle body will continually change in an unpredictable fashion as the vehicle traverses hills in various directions, as the vehicle travels around curves in the course, and as the vehicle changes speeds. The pitch and roll of the vehicle body, in combination with the presence of the force of gravity, dramatically affect the measured acceleration values.
One approach for eliminating body roll as a source of error has been to provide a horizontally stabilized reference platform, typically by means of one or more gyroscopes, which keeps reference axes of the accelerometers independent of the roll and pitch motions of the vehicle body. Such a system is disclosed, for example, in U.S. Pat. No. 3,094,877, issued June 25, 1963. This approach has several disadvantages, not the least of which is the high cost of gyroscopic instruments. Additionally, since the forward and lateral measurement axes are continually maintained in a truly horizontal plane, the acceleration measurements accurately reflect the acceleration experienced by the vehicle in the plane of the road surface only when the vehicle is driven on a perfectly flat, level track. Acceleration data from such horizontally stabilized accelerometers are not very meaningful when the vehicle is driven over terrain that is sloped in any manner.
Consider, for example, a car traveling a banked circular track at the so-called hands-off speed, at which the downward force of gravity is counteracted by the centrifugal force arising from the circular path. Under these circumstances, there is no lateral force acting upon the vehicle tires, yet a horizontally stabilized accelerometer system would provide an output indicating centripital acceleration.
As a second example, consider a car at rest on a banked surface. A leveled accelerometer would measure zero lateral acceleration, yet the tires must sustain a lateral load of Mg sin (.theta.), where Mg is the weight of the car and .theta. is the inclination of the track. As stated previously, for drivers, vehicle designers, tire manufacturers, and others interested in measuring vehicle acceleration forces, acceleration data is most meaningful if measured in the plane of the road surface. Such acceleration data corresponds to the forces acting at the interface between the tires and the road surface and is valid for any track path or inclination. Analysis of such data can yield insight into tire performance, driver expertise, and car capability in forward acceleration, braking, and cornering that has applicability to any road surface or configuration.
Various prior art acceleration measuring systems have dealt with the problems encountered in measuring acceleration in an axis or coordinate system which is not aligned with the axis or coordinate system of interest. One such system is disclosed in U.S. Pat. No. 3,094,877, issued June 25, 1963, to Gold. The Gold patent discloses an acceleration measuring system for an aircraft which measures the acceleration of the craft moving down a runway in a direction parallel to the direction of movement. Acceleration measurements are made by a pair of accelerometers which are aligned parallel to and perpendicular to the longitudinal axis of the airplane, respectively. The Gold system utilizes a simple analog circuit which provides an estimation of the pitch of the airplane based upon the deviation in the nominal vertical acceleration from an expected 1g reading. The Gold system is incapable of measuring a lateral acceleration and, additionally, is subject to errors due to bumps in the runway surface. Finally, the Gold system requires that the accelerometers be precisely aligned with the longitudinal axis of the aircraft.
Other similar acceleration measuring systems are disclosed in U.S. Pat. No. 3,713,343, issued Jan. 30, 1973, to Segerdahl et al; U.S. Pat. No. 3,712,109, issued Jan. 23, 1973, to Haken; U.S. Pat. No. 3,272,972, issued Sept. 13, 1966 to Yamron et al; and "Strap Down Navigation Technology: A Literature Survey", by Garg et al, J. Guidance and Control, Vol. 1, No. 3, May-June 1978, pages 161-172. None of these acceleration measuring systems relate to deriving forward and lateral acceleration data in the plane of the surface over which a vehicle moves by measurement of acceleration with accelerometers in vehicle body which is subject to pitch and roll.
Accordingly, it is seen that there is a need for improved acceleration measuring system which accurately determines the forward and lateral acceleration of a vehicle in the plane of the road or track surface from acceleration measurements taken in three nominally orthogonal axes defined with respect to the vehicle body.